Clustered dot and line multilevel halftoning for electrographic color printing

ABSTRACT

A method is disclosed for rendering monochrome or colored continuous tone images by a system having restricted continuous tone rendering capabilities, such as electrographic printers capable of printing more than two density levels on each addressable micro dot. A method for preferred halftone dot growth is described, starting from isolated dots arranged along base lines and auxiliary lines, evolving to high density lines along the base lines and approximating full continuous tone rendition for high density regions. Preferred arrangements for the orientation, spacing and absolute location of the base lines and auxiliary lines are disclosed and a method to generate preferred arrangements. A moire free combination of three halftone images is described for the reproduction of color images. A method to select a restricted set of energy levels to obtain linear reflectance response is disclosed. Halftone cells are arranged in supercells to improve the density resolution of the system.

This is a division of application Ser. No. 08/430,081 filed Apr. 27,1995.

FIELD OF THE INVENTION

The present invention relates to image rendering systems having thecapability to render consistently only a restricted amount of densitylevels in a black and white or colour reproduction of a continuous toneimage. More particularly the methods of the invention can be used in anelectrographic printer having multilevel capabilities.

BACKGROUND OF THE INVENTION

The reproduction of continuous tone images is traditionally done by fullcontone reproduction such as colour photo prints or by binary halftoningtechniques such as colour offset printing. Electrographic printing,where a latent image is formed by static electricity that is locallydischarged to form graphical representations, has an important membercalled electrophotographic printing. In electrophotographic printing,the static energy is partially removed by a directed light beam.Electrophotographic printers have traditionally a capability of binaryprinting. The illusion of continuous tone images is reached by binaryhalftoning techniques. Every addressable spot on the output, furthercalled micro dot, can get a high density corresponding with full tonercoverage or a low density corresponding with the absence of toner.

Recently, electrographic printers have also limited contonecapabilities. That means that the amount of toner per micro dot can bemodulated more continuously, such that the micro dot--afterrendering--can have apart from a low density and a high density alsosome mid densities. The density level can be regulated by an energylevel that is applied to the micro dot by an output device. Agfa-GevaertN.V. from Mortsel Belgium markets such an electrophotographic printerunder the trade name Chromapress. This is a duplex colour printer (cyan,magenta, yellow, black) having a resolution of 600 micro dots per inchproducing 1000 A3 pages per hour. Per micro dot, 64 energy levels can beselected. The output device can be also a thermographic printer, inkjetprinter, more generally an electrographic printer etc. The problem withthe mid densities is that these are not stable as a consequence of thephysics of the electrographic process. By instability is meant thatthere is not a one to one relation between the energy level applied tothe device and the density level obtained on the reproduction. Thedensity level of a first micro dot is strongly dependent on the energylevel applied to the micro dots in the direct neighbourhood of the firstmicro dot. Therefore several methods have been proposed to enhance thestability of the micro dots. This can be done up to a certain limitdependent on the density level. An important aspect of the remaininginstability is that not enough density levels per micro dot can berendered. Therefore a technique related to binary halftoning must beused, which is called multilevel halftoning. A problem with halftoningis that the spatial resolution is decreased to improve the densityresolution. Another problem is that internal moire can show up due tothe interaction between the micro dots and the halftoning pattern. Theseproblems have been addressed in WO-A-93 26116, for multilevel halftoningof images having one colour component. FIG. 7 of that applicationdiscloses a 3-bit grey halftone dot layout according to a mixed dot typeembodiment. For low output densities, isolated halftone dots appear on abackground. The halftone dots comprise microdots having two differentdensity levels. For higher output density levels, isolated bands appearand for the highest densities, maximum two different density levels arepresent in each halftone cell.

However, if different colour components are printed on top of each otherto get colour reproductions, colour moire can occur between thedifferent components. This problem is not addressed in the abovementioned application. Colour moire or inter image moire is differentfrom internal moire, as will be described below.

EP-A-0 370 271 discloses the formation of halftone dots to preventrosette moire and color shift, but is related to binary halftoning.Problems of stability of micro dots and aliasing of line structures byuse of multiple levels are not addressed in this application.

OBJECTS OF THE INVENTION

It is a first object of the invention to render images with a consistentand predictable density on the reproduction.

It is a second object of the invention to optimise the spatialresolution while keeping the density resolution as high as is necessaryto guarantee the impression of continuous tone image reproduction.

It is a third object of the invention to eliminate internal moire andinter image moire.

It is a fourth object of the invention to make the reproduction obtainedfrom the combination of several colour components less registrationdependent thereby avoiding line structure aliasing.

SUMMARY OF THE INVENTION

In accordance with the present invention, a colour or multi tonereproduction is disclosed comprising at least three halftone imagesprinted on top of each other, wherein each of said images is rendered ona recorder grid of micro dots and comprises isolated halftone dotswherein:

the area outside the halftone dots has a minimum background densityD_(min) ;

the halftone dots include at least one micro dot having a density levelD₁ and at least one micro dot having a different density level D₂, D₁and D₂ both higher than D_(min) ;

the centers of the halftone dots are arranged along a first set ofparallel equidistant base lines having a first orientation and along asecond set of parallel equidistant auxiliary lines having a secondorientation different from the first orientation;

the points of intersection of any base line with any auxiliary line havean identical relative position with respect to the micro dot closest tosaid point of intersection.

The requirement that the halftone dots are isolated and composed of twodifferent densities makes that unstable micro dots can be stabilised bystable micro dots with higher density. The requirement of identicalrelative position reduces or eliminates the internal moire. Otheradvantages will become clear from the detailed description below.

In a lot of printing or image rendering devices, the density on thecarrier is obtained via a process in which the energy is modulatedspatially to obtain spatially varying densities on the carrier. Forthermography, thermosublimation, thermal transfer processes etc., energyis applied to a thermal head or the like. Usually, the more energy isapplied, the higher the density on the carrier. For electrophotography,a semiconductor drum is loaded by a negative voltage and illuminated bya light source to diffuse the charge, where toner particles must beattracted. Also here, the energy level of the light source impinging onthe drum is proportional to the density on the carrier. It is possiblethat the light source always gives a constant light power, but that theamount of energy on the semiconductor drum is modulated by deflection ofthe light source towards or away from the drum. It is possible to thinkof systems where an increasing energy level gives a lower density levelon the carrier. The embodiments set out for this invention can be usedalso for this type of devices.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described hereinafter by way of example with referenceto the accompanying figures wherein:

FIG. 1 is a schematic representation of a multilevel halftoning device;

FIG. 2 explains the details of a line modulation technique;

FIG. 3 gives some examples of constant grey images, modulated by a linescreen in different orientations;

FIG. 4 gives examples of constant grey images modulated by a combinationof line screens;

FIG. 5 shows the results of the combination of line screen modulationand full contone modulation;

FIG. 6 shows an elementary cell and supercell along with base lineorientations for a preferred combination of three screens according tothe present invention;

FIG. 7 shows a test page for selecting a restricted set of energy levelsE_(j) ;

FIG. 8 is a graph giving the reflectance as a function of the energylevel;

FIG. 9 shows another test page giving equivalent results as FIG. 7

FIG. 10 is a graph showing the reflectance as a function of a set ofordered halftone cell types.

It has been found that the stability of isolated micro dots increases astheir density increases. If no energy is applied to the micro dot,generally no toner will be attracted to the location of this micro dot.If the maximal energy is applied, toner particles will totally cover themicro dot. If a low energy level is applied, the amount of tonerparticles can fluctuate randomly between broad limits. The more tonerparticles are deposit on the micro dot, the higher the density reachedon this particular micro dot. Moreover, it has been found that microdots, getting a low energy level, behave differently depending on theenergy levels applied to micro dots in their neighbourhood. Micro dotsgetting a high energy level render more consistently the same density,independent from the neighbouring micro dots. Further research revealedthat micro dots, getting a low energy level, become more stable whenthey are adjacent to a micro dot with high energy level. By adjacency ismeant that the micro dots touch each other side by side or by a cornerpoint. Therefore it is advantageous to divide the printable area inhalftone cells, each cell comprising the same amount of micro dots andredistribute the density of the individual micro dots in one halftonecell such that the average density--averaged over the micro dots of onehalftone cell--approaches the average density required for thereproduction of the image. A low average density can be reached by twoextreme arrangements:

(1) one or more micro dots in the halftone cell get the highest possibledensity; the other micro dots in the halftone cell get the lowestpossible density, except for one micro dot that gets an intermediatedensity so that the average density approximates the required density.

(2) all micro dots get essentially the same density such that therequired density is equally distributed over all micro dots.

The first method creates stable density levels but reduces the spatialresolution to the size of the halftone cells. The second methodpreserves the spatial resolution, but low densities will randomlyfluctuate between broad limits. Method (1) is absolutely necessary forlow density levels, while method (2) can be used for high densitieswithout quality degradation. Therefore, in low densities a first onemicro dot of a halftone cell is raised to a stable density, before asecond micro dot in the halftone cell can get a small density toincrease the average density of the halftone cell. From a certainaverage density level, the micro dots surrounding the first stable microdot can get an increasing density. This means that in low densities,only the first micro dot in each halftone cell will be visible. Inhigher densities, this first micro dot and micro dots clustered aroundthis first micro dot will be visible. Because all halftone cells arearranged in the same way and repeated periodically in two dimensionsover the whole reproduction, one will notice in low density regions aregular mesh of spotlike zones.

As the average density increases, the area of the micro dotscontributing to the average density increases. The shape of theseclustered micro dots can be round, square, elliptical, elongated etc. Wehave selected an elongated shape evolving towards a line screen, becausea line screen makes the combination of colour components in a colourreproduction less registration dependent. This means that in mediumdensities equally spaced parallel lines of micro dots having a highdensity are visible. Adjacent to these lines, the micro dots have alower density. The stability of the latter micro dots is secured by thehigh density micro dots along the parallel lines.

When the density gets higher, the average density can be more and moreequally distributed over all micro dots constituting the halftone cell.This has the effect that the space between the parallel lines getsfilled with micro dots having a higher density and that the lines seemto disappear.

The above described arrangement of the micro dots in a halftone cell canbe obtained by the combination of two line screens. A line screen is ascreen for which the density along a fixed direction, called the screenangle, remains the same for all points along a line in that direction.The density varies from low density to high density along every line ina direction orthogonal to the screen angle. For photomechanicalreproduction, the light reflected or transmitted by the originalcontinuous tone image is directed to the line screen. The light isattenuated proportionally to the local density of the line screen, andthe attenuated light is directed to a photosensitive material. Thecombination of the two line screens can be realised by putting them ontop of each other with different screen angles or by applying the firstline screen, developing the photosensitive material and then modulatingthe light from this photosensitive material by the second line screenunder a different angle. This will render small dots in the low densityregions, line structures in the medium densities and a continuum in thehighest densities.

The same principles can be applied to characterise formation of themicro dots in a halftone cell. In electronic imaging this formation canbe controlled by a set of N-1 threshold matrices, each matrix having Melements, or by a set of M pixel tone curves, each pixel tone curvehaving L entries. The value N represents the number of energy levels.For a binary system, N equals 2 and one threshold matrix can describethe evolution of the micro dots within one halftone cell. For a systemwith 16 energy levels, N=16. The value M represents the number of microdots in the halftone cell. For a square halftone cell with 4 pixels and4 lines of micro dots, M=16. The value L represents the number ofdigital intensity levels I_(x),y by which the continuous tone imageinformation for location (x,y) on the carrier or one component forcolour images is represented. For eight bit systems, L=256, i.e. thegrey value of a pixel of the continuous tone image can be represented bydigital values from 0 to 255. The representation of the halftone cell bypixel tone curves gives more flexibility than the representation bythreshold matrices, because threshold matrices force that the energylevel for any specific micro dot does not decrease when the averagedensity level of the halftone cell increases. Moreover, pixel tonecurves are faster to convert a pixel level to an energy level. Thisoperation can be done by one look up table operation once the locationof the micro dot relative to the halftone cell is known. It is obviousthat a set of threshold matrices can be converted to the correspondingset of pixel tone curves and that pixel tone curves that are neverdescending can be transformed to a set of corresponding thresholdmatrices.

In FIG. 1 is shown how the transformation of a continuous tone image 26to a multilevel halftone image 30 is realised. The carrier 21 is dividedinto micro dots 22. Every micro dot 22 on the carrier 21 is individuallyaddressable by the rendering device 23 by an address (x,y). Dependent onthe required size and orientation of the reproduction, the resolution ofthe rendering device and the original resolution of the image datarepresenting the original image, the pixels of the continuous tone imageare geometrically rearranged by techniques well known in the art toobtain an image 26 of pixel data 29, each having a location (x,y) and anintensity value I_(x),y. This geometric rearrangement can happen beforethe conversion to a multilevel halftone image or during the conversion,as the intensity values I_(x),y are required. This operation makes thatfor every micro dot 22, there is one input pixel 29.

All micro dots on the carrier are organised in halftone cells 31. InFIG. 1 each halftone cell is composed of sixteen micro dots 22. A clockdevice 28 generates a clock pulse for every micro dot. At every clockpulse, an address generating device 27, coupled to the clock device 28,generates the coordinates (x,y) for the next micro dot 22 to be imaged.This address (x,y) addresses a pixel 29 of the continuous tone imageinformation 26 and sends its intensity level I_(x),y to the tone curvetransformation unit 25. This unit also receives the address (x,y) andrelates this address (x,y) to a micro dot element number i within ahalftone cell. For this example, i ranges from 1 to 16, because thereare sixteen micro dots per halftone cell. Each micro dot i has a pixeltone curve L_(i) associated that transforms the intensity level I_(x),yto an energy index number j, corresponding to energy level E_(j). Aswill be discussed later, the energy index j can take a reduced amount ofnumbers, to address a restricted set of energy levels E_(j). A table 36containing these energy levels can be indexed by index j to give energylevel E_(j). That energy level E_(j) together with the address (x,y) issent to the rendering device 23, which results in a density written onmicro dot 22 on the carrier 21 at location (x,y).

The way the pixel tone curves are filled with values will determine thelook of the reproduction. We make use of the concept of line screens tofill up values in the pixel tone curve elements. The procedure isdescribed here in accordance with FIG. 2. For each constant intensitylevel I_(x),y, we generate one value for each pixel tone curve L_(i)within a halftone cell. We select a set of equidistant parallel linese.g. 41, 42 that gives an identical line pattern in every halftone cell45. The distance, measured along a line orthogonal to the parallellines, between every two adjacent lines is D. For a given intensityI_(x),y, we define bands 43, parallel to and centered along the parallellines, having a width W dependent on the intensity level I_(x),y.

The width W is D for the lowest intensity level;

The width W is 0 for the highest intensity level.

The width W of each band is further a decreasing function of theintensity level I_(x),y, i.e. W is proportional to (1-I) if I is thelinearly normalised value for I_(x),y such that I has values between 0and 1. The clause "y varies proportional to x" means that y is anascending function of x, i.e. whenever x increases, y does not decrease.For this specific case, for the highest density the bands touch eachother and for the lowest density there are no bands at all. As can beseen from FIG. 2, each micro dot 44 is covered by one or two bands,depending on the width of the band. It is now the area of this micro dot44 covered by the bands 43 that gives the amount of density allotted tothe specific micro dot. Is the micro dot fully covered by one or morebands, then this micro dot will get the highest possible density. If themicro dot is not covered by any band, then the micro dot will get thelowest possible density. If the micro dot is covered for 50% of its areaby the bands, then it will get an intermediate density between theminimum and maximum possible density. This way we can get line patternsas shown in FIG. 3. On a density scale from 0 to 255, resulting linepatterns are shown for a density of respectively 15, 36 and 98. On theleft side such a line pattern is shown for a line orientation of about166°, on the right side the line orientation is 45°. This type ofone-dimensional modulation gives improved density stability of images.Moreover, the aliasing that can be expected to happen when a line with acertain orientation is imaged on a discrete grid as the one formed bythe micro dots, is smoothed by the electrographic process itself, thattends to smear out the toner particles along a band along the line. Thetoner particles are concentrated along this line and the backgrounddeposition further away from the line center is diminished. This has theeffect of edge enhancement, originating from the electrical field in thefinite developing gap. This method has the effect that the requireddensity is "concentrated" along the parallel lines, giving the lowestdensity to micro dots more distant from said lines.

A further improvement can be reached by repeating the above step withlines having a second direction not coinciding with the first directionof the first lines. The density values assigned by the previous methodto the micro dots are called V₁ and are now used to be modulated by thesecond set of lines. The width of the second set of bands is nowproportional to V₁. If V₁ is zero, then the bands will have width zero;if V₁ is maximal--i.e. the area of the micro dot was totally covered bythe first bands--then the second bands have a width equal to the spacingof their center lines. Again for the micro dot having value V₁ assigned,the area covered by the second bands is measured, giving a value V₃.This value V₃ is zero if the second bands do not cover any part of themicro dot. This value V₃ is maximal if the micro dot is totally coveredby one or more of these second bands. This second modulation"concentrates" the previous line-wise densities towards the points whereboth lines cross each other. This can be seen in FIG. 4, where everyfigure corresponds to the one of FIG. 3, where the extra line modulationis added. It is obvious from this figure that for low densities (e.g.15) a periodical repetition of spots appears. For a higher density (e.g.36) these spots get elongated and start connection to lines. For evenhigher densities (e.g. 92). the lines are clearly visible and start tofade out to a continuous density all over the reproduction.

Because for higher densities the micro dots are more stable, thedensities can be distributed more equally over the micro dots. Thereforethe method can be modified in a sense that a homogeneous densitydistribution is added to the line-wise or dot-wise distribution. Thehomogeneous distribution gains more importance for higher densities.Therefore, from the value V₁ we can derive a value V₂ that is a linearcombination of the line-wise modulated density V₁ and the homogeneousdensity distribution:

    V.sub.2 =(1-W.sub.1)*V.sub.1 +W.sub.1 *(1-I)

W₁ is a positive weight factor, not bigger than one. I in (1-I)corresponds to the normalised form of the intensity I_(x),y. This valueV₂ is now used to define the width of the second bands. Another valuefor V₃ will now be obtained whenever W₁ is different from zero. The samemethod can be applied to the value V₃, obtained from the second set ofbands for the specific micro dot, giving a modified value

    V=(1-W.sub.2)*V.sub.3 +W.sub.2 *V.sub.2

It is obvious that the first method is a special case of this modifiedmethod, if W₁ and W₂ are selected to be zero.

In FIG. 5, the first row shows what happens if the first method isapplied with both weights zero. This figure is effectively correct forlow densities. For higher densities, the band like structure becomesmore apparent. The diamond common to the two bands is imaged at highdensity by this operation. The second row shows what happens if thefirst weight W₁ is taken zero and the second taken one. A line screenresults. The last line shows what happens if both weights have anintermediate value. It has been found that the best choice for theweights W₁ and W₂ is increasing with the density. The higher thedensity, the more the distribution over the halftone cell is allowed tobe homogeneous.

The value obtained for V can now be mapped to the available energylevels E_(j). Preferentially, a limited set of energy levels is selectedand fixed ranges of V-values [V_(j),V_(j+l) ] are mapped to one energylevel E_(j). If the number of selected energy levels is 16, then fourbits suffice to index these energy levels. The pixel tone curves canthen transform the input levels I_(x),y to a four bit value. UsuallyI_(x),y is represented by eight bits. This choice gives a saving of 50%memory. In electrographic devices where the energy levels are lessstable, it can be advantageous to allocate only four energy levels,reducing the number of bits to represent one micro dot to 2. Otherdevices having more contone capabilities can output reproductions withincreased quality if the number of energy levels is taken 64, requiringsix bits per micro dot.

The choice of the angle of the parallel lines and the distance betweenthese lines is first of all restricted by the requirement that theselines have to cover all the halftone cells in the same way. Moreover, ithas been found that more restrictions are necessary to avoid internalmoire. Internal moire is due to the interaction of the screen--in thiscase the line screen or the combination of both line screens--with therecorder grid or the micro dots. The preferred arrangement of theisolated spotlike zones--called halftone dots on the analogy of binaryhalftoning--for this invention is such that they form a periodicalstructure. More precisely, they are arranged along two sets ofequidistant parallel lines. Since for the highlights, one single smalland isolated halftone dot comes up per halftone cell, the relativeposition of the halftone dot with respect to the recorder grid or themicro dots should be equal for all halftone cells. The first set ofequidistant parallel lines is called the base lines, the second set ofparallel lines, having a different orientation and thus intersecting thebase lines, are called the auxiliary lines. Because the base lines andauxiliary lines go through the midpoints of the halftone dots, thecenter of the halftone dots is situated at the point of intersection ofa base line and an auxiliary line. It has been found that--in order toavoid internal moire--these points of intersection or the centers of thehalftone dots must be situated on points that have always the samerelative position or "spatial offset" with respect to the closest microdot. In other words, if all points of intersection are translated overexactly the same distance such that one point of intersection coincideswith the center of a micro dot, then all points of intersection coincidewith the center of a micro dot. We make here the difference between baselines and auxiliary lines, because at medium image densities the microdots with highest density are arranged along these base lines. Among themicro dots along these base lines there can be also be a densitydifference. In that case, the micro dots with highest density aresituated closest to the auxiliary lines.

This type of arrangement of halftone dots, gradually evolving to linepatterns can be used for monochrome images. A combination of thesearrangements can be used for multi tone reproductions. In that case onehalftone image having one colour will be superimposed on one or moresuch images having a different colour. It is known from binaryhalftoning that two halftone images can interfere with each other andproduce moire. When a third halftone image is superimposed on the set oftwo, secondary moire can arise. It has been found that these types ofmoire can also be produced by multilevel halftone images if the dot orline arrangement for each individual halftone image and the combinationis not selected adequately. Specially for colour reproductions composedof three halftone images: a cyan, magenta and yellow component orreproductions composed of four halftone images: a cyan, magenta, yellowand black component, it has been found that at least three of theindividual components preferably have the properties for the monochromehalftone image as discussed above. In that case, there are at leastthree sets of base and auxiliary lines. Like in binary halftoning suchas offset printing, only three of the four separations of cyan, magenta,yellow and black are given high weight in the optimisation of thescreen. Therefore we emphasize on the role of three separations withtheir accompanying screen geometry. Each set of base and auxiliary linescan be selected from a set of three or more generic lines. Every set ofbase and auxiliary lines will produce a set of points of intersection.The relative position of these points of intersection relative to theclosest micro dots can be established for every set. In a preferredembodiment, this relative position must be equal for each of at leastthree colour components.

Also the orientation of the base lines plays an important role whenthree multilevel halftone images are combined to produce multi tone ormore specifically colour images. This is especially true for the midtone areas of the image, where line-wise structures along the base lineshow up. First of all it is important that the base line of one colourcomponent is not parallel to any of the base lines of the other twocomponents. Three base lines selected from the three halftone images,such that they have no common point of intersection, form a triangle. Inorder to have quite similar screen rulings or line rulings and thus alsoa comparable spatial resolution for the three components of the colourimage, it is preferred that this triangle has no obtuse angle.Implementations where the deviations between the spatial screen rulingsof the different colour separations are minimal, have the advantage thatphysical processes such as dot gain are similar for the differentseparations. Especially in electrography with finite gap magnetic brushdevelopment, the development response will be different for screens ofdifferent spatial rulings. Geometries as presented here based ontriangles without obtuse angle, will therefore benefit from equaldevelopment response. Even more preferable, the triangle should approachan equilateral triangle. These conditions restrict the relativeorientation of the base lines for different multilevel halftone imageswith respect to each other.

Furthermore, also the absolute orientation of the base line and theauxiliary line plays an important role in the quality of a singlemonochrome multilevel halftone image, and thus also in the quality of amulti tone image composed of a set of halftone images. It is preferredthat the base line is not horizontal nor vertical. If the base line ishorizontal, then the tone curves of micro dots along horizontal linesare equivalent in the first step of the production process of them. Thismeans that the micro dots on the same horizontal line get the samedensity allotted, equally distributed over these micro dots, and microdots not on the same horizontal line get densities allotted for anotherrange of image intensity levels. The use of contone according to themethod described above is especially beneficial for sloped lines.Therefore, to benefit from the method and in order to keep thecharacteristics of the different colour separations the same, it ispreferred to chose screen angles different from 0° and 90°.

Normally, if four colours are used for printing, then the cyan, magentaand black component have to obey the rules sketched above for optimalrendering. The yellow component is less critical, first because it looksless dense to the human observer and because it has less sideabsorptions in the visual band. We have found that for the yellowcomponent the halftone cell structure for the black component can betaken and be mirrored along a horizontal axis or a vertical axis or onefixed point or any sloping line at 45°.

A method has been devised to find optimal combinations of angles anddistances for a combination of three multilevel halftone images, suchthat they obey the restrictions sketched above. The method givesdistances expressed in micro dot units. The effective screen ruling canbe obtained by dividing the recorder grid resolution by the distancebetween the base lines expressed in micro dot units.

The method finds three generic lines L1, L2 and L3 as shown in FIG. 6.

L1 is selected as the base line B1 for the first halftone image.

L2 is selected as auxiliary line A1 for the first halftone image.

L2 is selected as the base line B2 for the second halftone image.

L3 is selected as auxiliary line A2 for the second halftone image.

L3 is selected as the base line B3 for the third halftone image.

L1 is selected as auxiliary line A3 for the third halftone image.

As shown in FIG. 6, the orientation of L1 is given by a first vector V1from the origin (0,0) to point P1 (X₁,Y₁). If the origin is situated onthe center of a micro dot, the point P1 must be situated also on thecenter of another micro dot. This means that X₁ and Y₁ have integervalues. This restricts the choice for X₁ and Y₁ dramatically. Thischoice is further restricted by the screen ruling that is aimed for.Next all possible positions for a second point P2 with coordinates(X₂,Y₂) are tested. The set of points P2 is restricted by the fact thatagain both X₂ and Y₂ must have integer values and that the length L₂ ofthe vector P2 given by

    L.sub.2 =sqrt(X.sub.2.sup.2 +Y.sub.2.sup.2)

must be not too different from length L1 of the vector P1, given by:

    L.sub.1 =sqrt(X.sub.1.sup.2 +Y.sub.1.sup.2)

From these two vectors P1 and P2, we can derive a third vector P3=P2-P1,with coordinates (X₂ -X₁,Y₂ -Y₁). We take now the first generic lineparallel to P1, the second generic line parallel to P2 and the thirdgeneric line parallel to P3. The distance between generic lines L1 isthe orthogonal distance of P2 to vector P1. The second generic line istaken parallel to P2. The spacing between generic lines L2 is theorthogonal distance of point P1 to vector P2. The third generic line L3is taken parallel to vector P3. The spacing between lines L3 is theorthogonal distance of point P1 or P2 to vector P3 through the origin.It is obvious that the spacing between the generic lines is dependent onthe length of P1, P2 and P3. If we want equal spacings and thus equalline rulings, the length of these vectors must be equal. In other words,the differences L₂ -L₁, L₃ -L₁ and L₂ -L₃, where L₃ is:

    L.sub.3 =sqrt(X.sub.3.sup.2 +Y.sub.3.sup.2)

must be minimal. Therefore, for every possible vector P2, the followingmetric is computed:

    M=[(L.sub.1 -L.sub.2).sup.2 +(L.sub.2 -L.sub.3).sup.2 +(L.sub.3 -L.sub.1).sup.2 ]/A.sup.2

with A=area of the triangle (0,0), P1, P2. The vector P2 that minimizesthis metric M is taken as the best candidate for the vector P2. Anotherpreferred way of selecting the best base lines is by listing allcombinations (X₁,Y₁), (X₂,Y₂), computing the area for the correspondingparallelogram built upon those two vectors and selecting for each areaone combination that gives the best metric. We give here a table of somevectors P1, P2, P3 found by this method, along with the number of microdots or "cell area" in the halftone cell they define.

It is possible to restrict the (X₁,Y₁) combinations to those where X₁and Y₁ are both non-negative and X₁ ≧Y1. After processing, equivalentcell structures can be obtained by interchanging the role of X and Y,meaning mirroring about a line of 45°, by making X₁ and/or Y₁ negative,or any combination. The corresponding vectors P2 and P3 follow thetransformation accordingly.

    ______________________________________                                        X.sub.1 Y.sub.1                                                                             X.sub.2   Y.sub.2                                                                           X.sub.3 Y.sub.3                                                                           Area                                  ______________________________________                                        3       1     -2        1   1       2   5                                     3       -1    -2        -2  1       -3  8                                     4       -1    -3        -2  1       -3  11                                    4       -2    -3        -2  1       -4  14                                    4       -1    -3        -3  1       -4  15                                    3       3     2         -4  5       -1  18                                    5       -1    -4        -3  1       -4  19                                    3       4     2         -5  5       -1  23                                    4       4     1         -5  5       -1  24                                    6       1     -3        -5  3       -4  27                                    5       -2    -1        6   4       4   28                                    6       -2    -2        6   4       4   32                                    6       -2    -1        6   5       4   34                                    7       -2    -2        6   5       4   38                                    6       -3    -1        7   5       4   39                                    ______________________________________                                    

The halftone cell is exactly the same for the three halftone images andis the parallelogram built upon the vectors P1 and P2. The vector P3 isthe shortest diagonal line for this parallelogram. In FIG. 6 the examplefrom the above table is shown where P1=(7,-2), P2=(-2,6) and P3=(5,4).Each elementary halftone cell covers 38 micro dots. These elementaryhalftone cells can be arranged in a supercell of 19×19 micro dots.

The method described above gives an example to generate pixel tonecurves, and results for every micro dot R_(i) (i=1 . . . M, M=number ofmicro dots per halftone cell) in a value V per entry I_(x),y (I=1 . . .L, L=number of possible intensity levels in the input image) in thepixel tone curve L_(i). More generally, we can state that V=g(x,y,I).Other functions g(x,y,I) can be established using other methods. Assaid, among all possible energy levels E offered by the renderingdevice, only a restricted set of N energy levels E_(j) are reallydistinct enough to render consistently different densities on theoutput. Suppose that the halftone cell covers M=11 micro dots, and thenumber of selected energy levels N=16, then it is theoretically possibleto generate M*N=176 different output density levels. Even if the energylevels E_(j) are optimally chosen, these output density levels will notbe equally spaced. If we have 256 input intensity levels, we must mapmostly two input intensity levels to one output density level, and dueto the non-linear spacing of these output densities, sometimes three oreven four input levels must be mapped to one output density level. Thiscan result in contouring and quality loss. The object is to guarantee acorrect grey rendering with a restricted amount of energy levels E_(j).

Therefore it is advantageous to take an amount of S halftone cellstogether to form a supercell. The number of micro dots in the supercellequals M*S. The higher S. the more different output densitylevels=averaged over the supercell=can be generated. The number S isselected such that the micro dots can be rearranged to form a squaresupercell. Each micro dot in the supercell has now a pixel tone curvedefined by the function g(x,y,I), that is periodical over allsupercells. It is obvious that every pixel tone curve with identicalg(x,y,I) values will be present S times in the supercell. Such identicalpixel tone curves are called equivalent pixel tone curves. Due tosymmetry of the halftone cell, it is possible that the halftone cellitself already contained equivalent pixel tone curves. The functionV=g(x,y,I) gives an order in which halftone cell types are formed togive increasing densities. A halftone cell type is the distribution ofenergy levels over the micro dots of a halftone cell. Each halftone celltype will give a specific average density, averaged over the halftonecell. The halftone cell can be an elementary halftone cell or asupercell composed of elementary halftone cells. To render the lowestdensity, all micro dots will get an energy level E₀. For the next higherdensity for the supercell, one micro dot must get an energy level E₁,while all the other micro dots keep energy level E₀. The micro dothaving the largest value for V=g(x,y,I) will be the candidate toincrease its energy level. Because the supercell is composed of Selementary halftone cells, there will be at least S candidates toincrease the energy level from E₀ to E₁, or more generally from E_(j) toE_(j+1). Thus, the function V=g(x,y,I) gives only a coarse indication orprimary sequence on which micro dot gets an energy increment. Whenconverting the values V=g(x,y,I) just by a kind of truncation to indexesj for E_(j), which is a type of quantisation of the merely continuousvalues for V, all micro dots belonging to equivalent pixel tone curvesare candidates. If no finer ordering is imposed, this quantisationresults in an error in output density. One way of assigning the nexthigher energy level to a pixel tone curve, is to list them in asequential order according to the elementary halftone cell where theybelong to. But this will always introduce the same pattern in thesupercell, which will give visual artifacts in the image. The errorfunction due to quantisation will contain low frequency components thatare visually perceptible. It is better to generate an error functionthat has high frequency components. This error function can be createdby superposition of a pattern e(x,y,I) on the function g(x,y,I)

    g'(x,y,I)=g(x,y,I)+e(x,y,I)

The function e(x,y,I) is preferentially selected such that it variesbetween zero and the minimum difference between different values of thefunction g(x,y,I) in any different points (x₁,y₁) (x₂,y₂). This meansthat the error function imposes a sequence on equivalent tone curves oridentical V=g(x,y,I) values, but the error function does not change theorder imposed by the function g(x,y,I).

In a preferred embodiment, the error function e(x,y,I) is not dependenton the intensity level I and as such e(x,y) is only dependent on thelocation (x,y) of the micro dot within the supercell. This way the samesubordinate sequence is imposed on all identical g(x,y,I) values,whatever the intensity value might be.

In a more preferred embodiment, the error function e(x,y) variesaccording to the sequential numbering in a Bayer matrix. A Bayer matrixis well known in the art (see e.g. "An optimum method for two-levelrendition of continuous-tone pictures" by B. E. Bayer in ProceedingsIEEE, International Conference on Communications, Volume 26, pages11-15, 1973) for cell sizes that have a width and height that is a powerof two. If the supercell size is not a power of two, we can define ageneralised Bayer matrix as one with a bigger size being the next powerof two and with a subsection taken that covers the supercell. It is alsopossible to define a function over integer values 1 . . . 8 that returnsthe traditional Bayer matrix for values 1, 2, 4 and 8, and that gives ageneralised Bayer matrix for other values. This function can be used togenerate a generalised Bayer matrix if the supercell size can bedecomposed in prime numbers lower than eight.

Another quality improvement can be reached when the Bayer matrix israndomized in the following way. Each smallest Bayer submatrix--this isa 2×2 matrix if the supercell size is a multiple of 2, this is a 3×3generalised Bayer matrix if the first prime factor for the supercellsize is 3, etc.--is randomized. That means that the sequence numbers arerandomly permuted. For a 2-2 submatrix, there exist 4!=24 permutations.A traditional white noise random generator can be used to generatenumbers from 1 to 24 to select randomly one of the possible permutationsfor every sub-matrix in the Bayer matrix. This has the advantage that noaliasing effects can be observed due to the screen ruling imposed.

The process for establishing the pixel tone curves can be summarized asfollows:

(1) Define supercells having micro dots R_(x),y

(2) Compute for every micro dot R_(x),y a pixel tone curve G_(x),y=g(x,y,I) according to a function that maps an input intensity level Ito an output density g(x,y,I) for the micro dot at location (x,y). Thevalue g(x,y,I) indicates the order in which the energy level E_(j) mustbe increased to energy level E_(j+1).

(3) Add an error function e(x,y,I) to each pixel tone curve G_(x),y toobtain G'_(x),y. This error function imposes a subordinate order forwhich micro dot increases its energy level from E_(j) to E_(j+1).

(4) Assign energy levels E_(j) or indexes j to the pixel tone curvesL_(x),y in the order as indicated by the function G'_(x),y.

As stated above, a restricted set of M energy levels must be selectedfrom all possible energy levels. It has no sense to use all possibleenergy levels because the variation of densities produced by one energylevel overlaps too much with the variation of the next possible energylevel. By selecting a restricted set of energy levels, the number ofbits in the bitmap representing the halftone image can be reduced. Thenumber of energy levels is chosen in function of the contonecapabilities of the output device. If the variation in density fordifferent energy levels is small, then the restricted set can contain abig amount of energy levels, typically 64. Normally 16 energy levels areselected. This has the advantage that the energy level for two microdots can be stored in one byte of eight bits. In systems with poorcontone capabilities, typically 4 energy levels will be selected.

Preferably the energy levels must be selected such that the next energylevel gives the same decrement in reflectance, for every energy levelE_(j). This can be achieved by the following method. First of all, arepresentative subset--e.g. 16--of all possible energy levels--e.g.400--is selected. In this example energy level 1, 25, 50, etc. could beselected. It would be also possible to select all 400 energy levels toaccomplish the following procedure. In that case, the subset is the fullset of available energy levels. As shown in FIG. 7, with every energylevel from the subset a patch 52 or 53 is output on the image carrier 51or on the unit where the latent image is converted to a visible image aswill be discussed further. The size of each patch is preferably 10 mmhigh and 10 mm wide. As such it can be easily measured by an integratingdensitometer. Every patch consists of small isolated zones 54. The zonesare spaced apart from each other such that they don't influence eachother. Preferably, there is a spacing of at least two micro dots betweeneach zone 54. The zone itself consists of a high density kernel 55, anda halo 56. The high density kernel is preferably one micro dot, imagedby the maximum energy level available. Also lower energy levels can beused for this kernel 55, as long as it is a stable energy level thatgives a stable density on its own and stabilises the density of itsneighbouring micro dots whatever energy level they have. The halo 56 isimaged by one energy level from the subset, e.g. E₁, E₂₅. Preferably,the width of the halo is one micro dot. As such, each zone 54 has ninemicro dots, the center of which is imaged at maximum energy level andthe other eight micro dots are imaged with the energy level from thesubset. Each patch 52 is thus imaged by three energy levels: the lowestenergy level in the background between the zones 54, a stable energylevel and an energy level E_(j) from the subset. On one carrier,different patches 52, 53 with different energy levels selected from thesubset can be imaged. After imaging, the reflectance R_(j) of each patch52, 53, . . . having energy level E_(j) (e.g. E₁, E₂₅, . . . ) ismeasured. This can be done by an integrating densitometer that will takethe average density of a large amount of zones 54 together with the lowdensity background. As shown in FIG. 8, the reflectance R can be plottedagainst the energy level E_(j). This curve has been obtained byperforming the above described measurements on the Chromapress systemmarketed by Agfa-Gevaert N.V. Both the reflectance and the energy levelare normalised to [0,1], by linear scaling and subtracting an offset.Moreover, the direction of the reflectance axis Norm₋₋ refl is inverted,giving an ascending function R=f(E). The measurement points areinterpolated or approximated by a piecewise linear, quadratic or cubiccurve, giving a continuous function R=f(E). Because we are interested tofind the energy levels that give equal decrements in reflectance, theinterval [R_(min),R_(max) ] is divided in subintervals [R_(k),R_(k+1) ],all having the same length on the reflectance axis. The number ofsubintervals is the number of selected energy levels E_(k) minus one. Ifwe want to select sixteen energy levels, the number of intervals will befifteen. Via inverse evaluation of the function R=f(E) in the pointsR_(k) delineating the subintervals, we find the selected energy levelsE_(k).

It has been surprisingly found that the same method applied to anarrangement as sketched in FIG. 9 gives the same results. Here, thepatch 61 on the image carrier 62 consists of parallel bands 63, spacedfrom each other at a distance such that there is no influence of thebands on each other. Each band 63 consists of a narrow central band 64and two narrow side bands 65 and 66. As in the previous embodiment, thecentral band 64 is imaged by a high energy level that gives a stabledensity, stabilising also the density of the micro dots in the sidebands. The side bands 65, 66 are imaged by energy levels E_(j) from asubset of the available energy levels. The rest of the procedure isessentially the same, as described in relation with FIG. 7 and FIG. 8.The same energy levels E_(k) are found. Moreover, if the bands arearranged such that on a full patch 61 the percentage of high densitymicro dots 64, the percentage of intermediate density micro dots 65 andthe percentage of lowest density micro dots are the same as for thespotlike zones 54 in FIG. 7, then the reflectances R_(j) measured arethe same. This proofs that all energy levels produce consistently onedensity level as long as they are stabilised by a neighbouring microdot.

It was found also that the reflectances R_(j) are dependent onenvironmental parameters like temperature, humidity, further dependenton the type of toner and paper used, the age of the drum etc. Thereforit is advantageous to repeat these measurements for different parametersand store the results E_(k) as a function of these parameters. If e.g.sixteen energy levels E_(k) are selected, then it is very easy to storesixteen sets of selected energy levels. The parameters named above canbe measured or kept track off, and whenever necessary, the energy leveltable 36 shown in FIG. 1 can be reloaded. This is a very fast operationthat can improve dramatically the reproducibility of the output fordifferent environments. This method to obtain a restricted set of energylevels E_(j) summarises the whole printing engine in just the sequenceof energy levels. The non linearities of the output device are correctedby this sequence. This method can preferentially be implemented in anautomatic image density control system. The patches described above canbe imaged on a specific location on the drum where the latent image isformed, toner is applied to this location, the location is illuminatedby a LED or laserdiode and the reflected light is measured by aphotosensitive sensor, that integrates over the patch.

Once the restricted set of energy levels has been selected and the orderfor increasing the energy levels in the different pixel tone curves isfixed, we must select exactly L energy distributions over the supercellor L halftone cell types from all the possible distributions offered. Lrepresents the number of intensity levels in the input image, and can betypically 256. Aspects due to the cell structure, such as dot gain canbe linearised by the next method. If the number of selected energylevels N is sixteen, and the number of micro dots S*M in the supercellis 225, then there are N*S*M=16*225=3600 halftone cell types numberedN_(j) in a sequence to bring the supercell from lowest density tohighest density. The order in which the energy levels are incrementedfor each micro dot can be given by the perturbated function G'(x,y,I) asdescribed above. It is now possible to output patches of 10 mm by 10 mmfor all 3600 halftone cell types in the sequence. The patch is overlaidwith supercells, and each supercell within the patch has exactly thesame distribution of energy levels, given by the sequence number N_(j).The reflectance of the patch composed of supercells imaged by halftonecell type N_(j) can be measured, giving a reflectance value R_(j). Themeasured reflectances R_(i) can be plotted against N_(j) as shown inFIG. 10. Also this curve has been obtained by performing the abovedescribed measurements on the Chromapress system. Here again both axesare normalised to the interval [0,1], and the reflection axis isoriented inversely. This gives on the R_(j) axis a point with minimumreflectance R_(min) and a point with maximum reflectance R_(max). Theinterval [R_(min),R_(max) ] is divided in L-1 equal subintervals,because it is an object to map the L intensity levels I_(x),y linearlyto a reflectance level R. This subdivision gives L-1 subintervals[R_(k),R_(k+1) ]. For each value R_(k) the value R_(j) that is closestto it is searched and the corresponding sequence number N_(j) isselected as a candidate for defining the energy distribution over thesupercell for intensity level I_(k) that will be mapped to reflectanceR_(k).

As the supercell gets larger, it becomes unpractical to list allarrangements N_(j) found by the function G'(x,y,I). Therefore, apreselection of a subset of all arrangements N_(j), preferablycontaining 16 elements, is made. It is possible to define as the subsetthe full set of all available halftone cell types N_(j). Again the plotas shown in FIG. 10 is made, but the number of measurement points isless dense. For every required reflectance R_(k), the index j is soughtsuch that R_(j) ≦R_(k) <R_(j+1). R_(j) corresponds with sequence numberN_(j) and R_(j+1) with sequence number N_(j+1),. Because in the previousstep we made that an increment in energy level gives a constantdecrement in reflectance level, and for every next element in thesequence one energy level of the complete supercell is incremented, wecan linearly interpolate between N_(j) and N_(j+1) to find the sequencenumber N_(k) that gives the reflectance R_(k) :

    N.sub.k =N.sub.j +(N.sub.j+1 -N.sub.j)*(R.sub.k -R.sub.j)/(R.sub.j+1 -R.sub.j)

The function R_(j) =f(N_(j)) is highly non-linear. Therefore, if onlysixteen samples of this function are measured, it is advantageous tomake a continuous interpolation or approximation by a piecewisenon-linear function, such as a cubic spline function. Approximation willfurther offer the ability to smooth out measurement errors.

If the selected subset of combinations is such that it lists allpossible combinations wherein the equivalent micro dots of differenthalftone cells in the same supercell always have the same energy level,then the difference in reflectance between the required reflectanceR_(k) and the obtained reflectance R_(j+1) can be used to determine thenumber of equivalent micro dots that must get an energy increment. Whichmicro dots will get the increment is determined by the error function orthe subordinate sequence given to the micro dots in the supercell.

It is obvious that this method can also be applied to elementaryhalftone cells, but the reflectance levels that can be really reachedwill be much coarser.

Although the present invention has been described with reference topreferred embodiments, those skilled in the art will recognise thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

We claim:
 1. A method for selecting a restricted set of N energy levelsfrom a possible set of energy levels, comprising the followingsteps:selecting subset E_(j) from the possible set of energy levels;marking for each selected energy level E_(j) from said subset aplurality of identical spotlike zones, isolated from each other by a lowdensity background, each zone comprising an internal zone of fixed highdensity marked by a stable energy level E_(s) and a surrounding zone ofdensity marked by said energy level E_(j) ; measuring the reflectanceR_(j) integrated over said plurality of identical zones partially markedby energy levels E_(j) ; dividing the range of measured reflectancesR_(j) in N-1 equally sized intervals (R_(k), R_(k) +1); finding anenergy level E_(k) for each reflectance R_(k) using the measuredreflectances and energy levels (R_(j), E_(j)); and defining therestricted set as being composed of the energy levels E_(k).